drm/i915: Make hpd debug messages less cryptic

Don't print raw numbers, use port_name() and tell the user whether it's
long or short without having to figure out what the other magic number
means.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>